This invention relates to an expandable fusion implant suitable for anterior approaches to the spinal column. The class of implements to which this invention pertains serve to stabilize adjacent vertebral elements, thereby facilitating the development of a bony union between them and thus long term spinal stability.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaptation is a relatively recent change, and as such has not benefitted from natural selection as much as have backbones held in a horizontal attitude. As a result, the stresses acting upon the human backbone (or "vertebral column"), are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvic or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures--discs--composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independently of each other, as well. The repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height relaxes tension on the longitudinal spinal ligaments, thereby contributing to varying degrees of spinal instability such as spinal curvature.
The time-honored method of addressing the issues of neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
While cancellous bone appears ideal to provide the biologic components necessary for osseous union to occur, it does not initially have the strength to resist the tremendous forces that may occur in the intervertebral disc space, nor does it have the capacity to adequately stabilize the spine until long term bony union occurs. For these reasons, many spinal surgeons have found that interbody fusion using bone alone has an unacceptably high rate of bone graft migration or even expulsion or nonunion due to structural failure of the bone or residual degrees of motion that retard or prohibit bony union. Intervertebral prostheses in various forms have therefore been used to provide immediate stability and to protect and preserve an environment that fosters growth of grafted bone such that a structurally significant bony fusion can occur.
U.S. Pat. Nos. 5,505,732, 5,653,762, 5,665,122, and 5,683,463 describe different prior spinal implants. The implant shown in U.S. Pat. No. 5,483,463 is hollow and tubular, with communicating windows in the top and bottom surfaces. External ribs, which may be serrated, stabilize the implant once it is inserted between the vertebrae. In U.S. Pat. No. 5,665,122, an intervertebral cage is rendered expandable by a wedging mechanism. The degree of expansion is rather limited, however. U.S. Pat. Nos. 5,653,762 and 5,505,732 show shaft-type tools used for installing implants. The prior devices do not enable one to achieve great ranges of implant height.
Limitations of most present-day intervertebral implants are significant and revolve largely around the marked variation in disc space shape and height that results from either biologic variability or pathologic change. For example, if a disc space is 20 mm in height, a circular implant bridging this gap requires a minimum diameter of 20 mm just to contact the end plate of the vertebral bone. Generally, end plate disruption must occur to allow a generous bony union, meaning that an additional 2-3 mm must be added on either end, resulting in a final implant size of 24-26 mm. During implantation from an anterior approach (from the front of the body), excessive retraction (pulling) is often required on the great blood vessels which greatly enhances the risk of devastating complications such as vascular tears or thrombosis. Compromising on implant size risks sub-optimal stability or a loose implant, which has a greater chance for migration within or expulsion from the disc space.
Because of difficulty with retraction of vascular elements and inadequate anterior exposure, single cylindrical devices are often implanted to compensate for the inadequate exposure required to implant paired devices. A single cylindrical implant has the disadvantage of allowing rotation of vertebral elements around the cylindrical axis of the implant. This in turn results in a degree of lateral instability ("barrel roll") which may result in non-union at the fusion site.
To counteract this problem, surgeons have attempted to place a single cylindrical fusion device at an oblique angle across the disc space. While this eliminates some of the barrel roll effect laterally, movement is still possible and can result in non-union. Other surgeons have recommended an oblique angled implement backup up with posterior pedicle screws, but this approach has the distinct disadvantage of requiring a major secondary surgical procedure from the posterior approach to achieve this. In addition, a single intervertebral implant placed centrally contacts the weakest part of the vertebral endplate which means little endplate support is present to retain the implant in position. Frequently, these lone implants will sink or subside into the soft cancellous portion of the vertebral body above or below. This subsidence means that the annular tension provided by the implant is lost and instability at the segment with concomitant progression to non-union at the fusion site occurs. Non-union is a disappointing consequence of this occurrence, and frequently results in the need for further surgical procedures.
Obviously, it would be of value to have a single stand-alone implant that could be introduced anteriorly without the drawbacks of rotational instability about the cylindrical or longitudinal axis of the implant. It would also be desirable to have greater endplate support provided by the implant to prevent subsidence into the adjacent vertebral cancellous bone, and the resultant loss of stability consequent with this. Having an expandable implant which can adjust to variabilities in disc space height would be an added benefit.